Investigating chromosome pairing in bread wheat using ASYNAPSIS I.

Abstract

Pairing and synapsis of homologous chromosomes are required for normal chromosome segregation and the exchange of genetic material during meiosis. Pairing is defined as the recognition and alignment of chromosomes that occurs either pre-meiotically or during early prophase I to ensure that associations via synapsis and recombination occur only between homologues. Synapsis is the intimate juxtaposition of homologous chromosomes that is complete at pachytene following formation of a tri-partite proteinaceous structure known as the synaptonemal complex (SC). In yeast, HOP1 is an essential component of the SC that localises along chromosome axes during prophase I and promotes homologous chromosome interactions. Homologues in Arabidopsis (AtASY1), Brassica (BoASY1) and rice (OsPAIR2) have been isolated through analysis of mutants that display decreased fertility due to severely reduced synapsis of homologous chromosomes. Analysis of these genes has indicated that they play a similar role to HOP1 in pairing and formation of the SC through localisation to axial/lateral elements of the SC. In this study, we have characterised the bread wheat homologue of HOP1, TaASY1, and its encoded protein. The full length cDNA and genomic DNA clones of TaASY1 have been isolated, sequenced and characterised. TaASY1 is located on chromosome group 5 and the open reading frame displays significant similarity to OsPAIR2 (84%) and AtASY1 (63%). In addition to OsPAIR2 and AtASY1, the deduced amino acid sequence also displays sequence similarity to ScHOP1, with all four proteins containing a HORMA domain. Transcript and protein analysis showed that expression is largely restricted to meiotic tissue, with elevated levels during the stages of prophase I when pairing and synapsis of homologous chromosomes occurs. Antibodies specific to TaASY1 were used in immuno-fluorescence microscopy and immuno-gold transmission electron microscopy to investigate the localisation of TaASY1 in meiotic cells. Immuno-fluorescence analysis initially detected ASY1 in pollen mother cells (PMCs) during meiotic interphase as foci randomly distributed over the chromatin. The ASY1 signal became increasingly continuous during leptotene, reflecting the changes occurring in chromosome morphology. Throughout zygotene, the signal became progressively more continuous, localising along the entire length of the axial elements as chromosomes synapsed. This signal appeared to persist until pachytene, before disappearing from the chromatin as the SC disassociated through late pachytene and early diplotene. The immuno-gold based electron microscopy displayed that TaASY1 localises to chromatin that is associated with both axial elements before SC formation as well as chromatin of lateral elements within formed SCs. Analysis of RNAi Taasy1 mutants was performed to further define the role of ASY1 in bread wheat meiosis. ASY1 localisation was disrupted in these mutants, with a diffuse and non-continuous signal observed through leptotene and zygotene. Feulgen staining of meiotic chromosomes displayed reduced synapsis during prophase I, as well as multivalents at metaphase I and abnormal chromosome segregation during anaphase I. These observations are consistent with the presence of homoeologous chromosome interactions. TaASY1 expression and localisation was also investigated in the bread wheat pairing mutant, ph1b. Quantitative real-time PCR (Q-PCR) revealed that TaASY1 is significantly up-regulated in ph1b, with greater then 20-fold expression compared to wild-type Chinese Spring, while maintaining the same pattern of expression as wild-type through progressive stages of meiosis. ASY1 localisation was significantly disrupted in ph1b, with irregular loading on axial elements during mid to late zygotene, indicative of abnormal chromatin remodelling and multiple axial element associations that have previously been reported in ph1b. Taken together, these results indicate that TaASY1 is essential for promoting homologous chromosome interactions during meiosis, and that impairment of ASY1 function in bread wheat meiosis results in reduced restriction of chromosome associations to homologues.